Compound eyes are the most abundant photoreceptive organs found in the animal kingdom. They are found in crustaceans (Crustacea) and insects (Hexapoda). These groups are nested within the phylum Arthropoda, arguably the largest group in the animal kingdom, and are characterized by their jointed appendages. Also included in Arthropoda are Myriapoda (centipedes and millipedes) and Chelicerata (spiders and horseshoe crabs). Rather than having compound eyes, myriapods and chelicerates have simple eyes with a single cuticular lens. Contrary to popular belief, these groups are not true insects and are thus separate lineages from insects and crustaceans. With an estimated appearance of 540 million years ago or older, compound eyes are incredibly complex and even unique to the species level. However, there are some conserved structures, including hundreds to thousands of independent ommatidia consisting of a corneal lens, crystalline cone, pigment cells, and photoreceptive cells within the rhabdom membrane. Each ommatidia works independently to capture light and send messages to the central ganglia (brain) to form an image so that the arthropod may navigate through its environment.

There are two general types of compound eyes that can be classified as:

 

1. Apposition compound eye: (most common)

The corneal and crystalline cone cells focus light on the rhabdom containing the photoreceptive cells. The crystalline cone allows light to be directed onto the photopigments and photoreceptive cells, triggering a photoelectric response that travels to the central ganglion for processing. A mosaic image is created, with each ommatidia contributing a small piece of the surroundings into a larger image, like a puzzle. The early Cambrian predator, Anomalocaris, possessed an apposition eye, dating this type of compound eye back at least 500 million years ago (Cronin, et. al., 2008; Meyer-Rochow, 2015).

 

2. Superposition compound eye: (clear-zone eye)

This type of compound eye is mainly found in nocturnal species such as moths, beetles, lobsters, shrimps, and more. It is characterized by a pigment-free gap between the cornea and cone and the photoreceptive cells within the rhabdom. Rather than each ommatidia independently functioning to create a mosaic of images, multiple ommatidia absorb light and are refracted into one photoreceptor cell. The clear-zone gap allows for the light to be refracted in this way. In low light, the pigment cells move into the clear-zone gap to sharpen the image and restrict the aperture. Ultimately, the superposition eye creates a blurry image with low resolution (Cronin, et. al., 2008; Meyer-Rochow, 2015).

 

 

 

 

When referring to the homology and non-homology of a character (an inherited trait or structure that appears and is present in a lineage), we are describing its relationship with other lineages. 

 

Homology: 

A homologous character is an inherited trait shared among different lineages and is derived from a shared common ancestor. Both of the split lineages possess the character, or a modification of the character, that was present in their shared common ancestor. The character does not have to appear exactly the same in both lineages, as divergence of species can lead to major modifications of a trait. However, the trait must be traced back to a common ancestor that is shared between the split lineages. For example, the bones of all tetrapods (which include humans) are homologous. This means that all tetrapods shared a common ancestor that developed 4 limbs. As the tetrapod lineage split and diverged through evolutionary time, major modifications to the bone structures occurred. However, we still share the same bones (humerus, ulna, radius, femur, etc.) as all other mammals, birds, reptiles, and amphibians that are within the tetrapod lineage. Although morphologically very different, the tetrapod bones are derived from a shared common ancestor from which we inherited them.

 

Non-homology:

A non-homologous character (also referred as analogous) is a similar trait between lineages that is not derived from a shared common ancestor and is often the result of convergent evolution. Convergent evolution occurs when distantly related lineages develop similar traits or structures that evolved independently of one another. This can be driven by similar ecological or environmental pressures. For example, the wings of birds and bats are non-homologous structures for flight. Birds are within the class Aves, and bats are within the class Mammalia, and their wing structures are very different. While birds evolved feathers to aid in flight, bats instead evolved thin membranous skin that stretches along the entirety of their elongated metacarpals and phalanges (finger bones). Although serving the same function, these structures evolved independently between birds and bats and are thus non-homologous structures as a result of convergent evolution.

Before diving into the debate over whether the compound eyes of insects and crustaceans are homologous or non-homologous, it is important to note that compound eyes did not just appear in evolutionary time as we see them today. Instead, the building blocks of the first eye possibly began evolving during the Precambrian period over 540 million years ago, when most organisms were microscopic or sessile. There was likely only a need for light-sensitive cells, especially for ancient corals living in symbiosis with photosynthetic zooxanthillae.

Once the Cambrian Explosion occurred, diverse mobile organisms evolved, including arthropods. Suddenly, there was a need for eyesight. Researchers have found the Pax-6 gene in Cambrian fossils, a gene that regulates major developments in modern-day taxa, like the eye. It could be possible that the Pax-6 gene evolved in the Precambrian period, allowing for the first light-detecting cells. Over millions of years, the light-detecting cells evolved into photoreceptive cells until image-forming eyes developed like those of compound eyes (Nilsson, 1996).

So, compound eyes are a product of a long evolutionary history of the first genes for light-detecting structures. Compound eyes did not simply appear in evolutionary history as the complex form they are today. 

Now, we can dive into the debate over whether compound eyes evolved once or independently in arthropods. In other words, compound eyes are homologous or non-homologous in insects and crustaceans.